1. Field of the Invention
This invention relates generally to the operation of a micro-electromechanical system (MEMS) device, particularly to such a device that includes a scanning mirror that operates at a resonant frequency.
2. Description of the Related Art
The scanning of optical beams by rapidly oscillating mirrors of a MEMS (micro-electromechanical system) assembly is an important aspect of devices such as displays and bar-code scanners. In many applications, the oscillations must be continuously maintained at a resonant frequency, which may be subject to destabilizing perturbations such as variations in ambient temperature and air pressure. These perturbations act by affecting the device characteristics that determine its resonant frequency. Clearly, a mechanism that would tend to stabilize operation at the resonant frequency of such a MEMS device would be of considerable importance.
The present MEMS is a mirror operating as a forced torsional oscillator. The force is applied to the mirror by means of a pulse train producing a time varying, periodic, pulsed electric field between the opposite ends of the mirror and electrodes of a fixed substrate.
Referring to FIG. 1A, (a)-(f), there is shown, schematically, a sequence of positions of such an oscillating mirror (6) resulting from the combination of an impressed series of periodic driving voltage pulses applied to the mirror by electrical contact with the voltage source (not shown) at its axis of rotation (5) of the mirror, together with a spring-like (torsional) restoring torque applied by the angular motion of the axis of rotation itself. The pulses produce a force between each of the opposite lateral ends (1) and (2) of the movable mirror and corresponding fixed ends (3) and (4) of the substrate electrodes. These electrostatic forces will vary with the angular position of the mirror (relative to its horizontal position in (a), for example) due to the finite surface areas of the moving mirror and stationary portions of the electrodes and their relative separations. The spring-like restoring force about the axis of rotation (caused, for example, by the physical twisting of the axis) tends to bring the mirror back to the neutral, horizontal position shown in (a), (c), and (e). However, perturbations caused by slight mechanical imbalances, will allow motion to be initiated, little by little, from a position at rest.
Each of the separate images (a)-(f) is a schematic representation of the position of the mirror at a slightly later time (see downward directed arrow of increasing time) and each of the numerals (1) through (6), are understood to represent the same features of the device in each successive illustration. Alongside the figures there is drawn a representation of a portion of a pulse train composed of square waves, that produce a driving voltage waveform that varies between zero and some nominal voltage, denoted HV. These pulses are progressively (in time) applied between the movable mirror and the fixed substrate, so that the voltage falls to zero at position (100), remains at zero until position (101) where it rises to HV, and then returns to zero at position (102).
Referring to FIG. 1B, there is shown schematically how a fixed light source, (35) can direct a beam of light (dashed line) onto the mirror (6) and have it reflect into an appropriately located photosensor (45). As the mirror rotates, the variation in angular position of the reflected beam, as detected by the photosensor, can be used to determine the corresponding amount of mirror deflection. This method, in fact, will be used to generate positional waveforms for the rotating mirror during the operation of the MEMS device.
Referring now to FIG. 1C, there is shown a schematic top-view of the same device of which FIG. 1A is a side-view. The device, which is shown here in an exemplary form, includes six identical mirror portions, labeled (10)-(15), affixed to a common shaft (5), which serves as an axis of rotation and a source of spring-like restoring torque. In FIG. 1A, the six mirror portions appeared as a single portion. The ends of each of the mirror portions (1) and (2), are interleaved between fin-like extensions of opposite ends (20) and (30) of electrodes fixed to the substrate of the device.
Returning again to FIG. 1A, (a), the mirror is shown in a horizontal position with its ends centered between the fins of the fixed substrate and there is a zero voltage at each end of the mirror. When a voltage appears on the mirror corresponding to the rise of the voltage pulse to V at (100), it is understood that a slight initial angular perturbation allows the voltage to move the mirror in a counter-clockwise direction to position (b), while the voltage remains at V.
Referring next to FIG. 1A (b), it is shown that the voltage at (101) has returned to zero, allowing the torsional force of the axis to return the mirror to its neutral position at (c). The remaining figures, FIG. 1A (d) to FIG. 1A (f) and beyond are a continuation of this periodic process.
The operation of the MEMS requires that it be forced into a resonant oscillatory mode by a periodic driving signal and then maintained in that mode during whatever ambient changes in temperature and air pressure occur to alter the resonant frequency of the structure. To achieve this object the system must be able to both continually measure the actual rotational frequency of the MEMS and to then make corrections to that frequency when it falls outside of acceptable resonance values. The prior arts listed below all include methods to achieve this object, but none are able to do so in a simple, efficient and cost-saving way.
Linden et al. (U.S. Pat. No. 7,485,485) discloses a closed loop system to control phase, amplitude and resonance frequency of a MEMS scanner.
Gibson et al. (US Patent Appl. 2005/0280879) describes operation of a MEMS scanner close to its resonance frequency by resonance frequency control.
Yazdi et al. (U.S. Pat. No. 6,985,271) uses capacitance to detect the position of a mirror and a closed loop feedback control.
Horsley et al. (U.S. Pat. No. 6,674,383) and (U.S. Pat. No. 6,933,873) varies and measures the position of an electrostatic actuator using a pulse width modulated (PWM) pulse train.
Turner et al. (U.S. Pat. No. 6,497,141) drives MEMS structures at parametric frequencies to permit precise switching between stable and unstable operation.
Hagelin et al. (U.S. Pat. No. 7,545,237) shows a MEMS device having a serrated tooth surface.
Amm et al. (U.S. Pat. No. 6,781,739) discloses a high frequency drive for MEM devices.
Hagen (U.S. Pat. No. 6,812,669) separates amplitude and wave shape inputs to a MEMS device to allow use of an inexpensive DAC to control the device.
Sprague et al. (U.S. Pat. No. 7,515,329) and (U.S. Pat. No. 7,442,918) drives a MEMS oscillator by applying torque to support arms.
Milanovic et al. (U.S. Pat. No. 7,428,353) provides MEMS device control using filtered voltage signal shaping.